1. Field of the Invention
The present invention relates to the preparation of liquid polyisocyanate compositions having enhanced stability and to the use thereof in the preparation of polyurethane foams having improved physical properties. More particularly, the invention relates to liquid polyisocyanates obtained by heating at high temperatures 4,4'-diphenylmethane diisocyanate (MDI) or mixtures containing MDI in the presence of tris(chloromethyl)phosphine oxide.
2. Prior Art
The preparation of liquid carbodiimide-containing organic polyisocyanates by heating an organic polyisocyanate in the presence of catalytic amounts of tris(chloromethyl)-phosphine oxide at a temperature of up to 120.degree. C. is taught in U.S. Pat. No. 3,761,502. Disclosed in this patent is the fact that use of this catalyst provides an improvement over prior art processes since its reactivity is such that it is possible to control the carbodiimide formation thereby enabling the preparation of liquid compositions having controlled amounts of carbodiimide linkages. One of the problems encountered in following the process of this patent is that the resulting compositions are not stable and over a period of time show decreasing amounts of isocyanate content. The present invention is directed to an improved process for the preparation of liquid carbodiimide-containing MDI employing tris(chloromethyl)phosphine oxide as the carbodiimide-promoting compound.